1. Field of the Invention
This invention relates generally to systems for conditioning air and specifically to control systems for maintaining a desired flow rate of conditioned air through at least part of the system regardless of the static pressure therein.
2. Description of the Background Art
In the past, various different techniques have been used in an attempt to flow air through a contained space of a system including air distribution systems for conditioning the temperature of the air with the rate of such air flow being related to the static pressure in the system. The rate of air flow (CFM--cubic feet per minute) through the air distribution system also affects the speed and torque of a motor used in the system.
One approach of the past involved the laborious task of matching the motor speed and torque with the proper fan to approximate the desired air flow rate for the particular contained space and static pressure of the particular air distribution system. However, this did not accommodate variations in the static pressure in the air distribution system caused by alterations in the system such as opening, closing or adjusting of a damper connecting a conditioned space in air flow relation with the system. In addition, other devices, such as filters and heat exchangers, may alter the static pressure within the duct system.
If the fan or blower utilized in such prior art systems was of the fan or blade type, a decrease in the static pressure acting on such fan resulted in an increase in the speed of the fan and the electric motor driving it. Conversely, if the static pressure on the fan was increased, the speed of the fan and electric motor decreased. Thus, the speed of the fans and electric motors utilized in the past varied inversely with a variation of the static pressure in the system.
Another prior art approach has been to compensate for the alterations in the speed of the fans and the electric motors by employing an apparatus for controlling the motor speed which required the calculation of constants specific for each apparatus and air distribution system combination. This approach further required signal comparison devices and motor current sensing devices. Additionally, this approach did not directly calculate the actual air flow rate in the air distribution system.